ABSTRACT for the 2009 Celebration of Research and Creative Activity


A Simplified Shack Hartmann Sensor to Demonstrate Wavefront Analysis

Shannon Hicks, John Noe and Anand Sivaramakrishnan, Laser Teaching Center,
Department of Physics and Astronomy.

In recent decades the technique of adaptive optics has revolutionized
ground-based observational astronomy. Due to the unavoidable turbulence in
the atmosphere wavefronts from distant light sources such as stars become
distorted and fluctuate many times a second. Thus when these objects are
viewed through a telescope the image becomes blurred and constantly shifts,
an effect astronomers call "seeing." Adaptive optical systems allow
astronomers to correct these wavefront distortions in real time using
deformable mirrors whose shape can be rapidly varied under computer
control.

An essential component of every AO system is a wavefront sensor to rapidly
evaluate the shape of the incoming distorted wavefronts. The commonly-used
Shack-Hartmann sensor consists of a 2-dimensional array of hundreds of tiny
lenses ("lenslets") that typically occupies an area smaller than a postage
stamp. Each lenslet creates a separate focal spot on a CCD camera mounted
behind the lens array. If the incoming wavefront is distorted (not a plane
wave) these spots of light shift in position by an amount proportional to
the distortion. Computer programs capture images from the camera and
determine the centroid shift of each spot and hence the overall shape of
the wavefront.

The goal of our project is to simulate and demonstrate the operating
principles of a Shack-Hartmann sensor and to gain experience with the
associated data analysis. Instead of an array of many lenslets, we use a
single "lenslet" that can be shifted to different positions using a
translation stage. This single "lenslet" is not actually a tiny lens but
rather a normal camera lens (16 mm focal length) preceded by a centered 500
micron diameter pin-hole aperture. The lens is mounted to our
Electrim-1000N CCD camera in the usual way. The camera software captures
images and saves the array of 8-bit pixel values as .tiff files which can
be converted to other formats as needed.

We are currently analyzing some preliminary data on a spherical wavefront
of 1.0 meter radius created by illuminating a 1.0 mm aperture with a
halogen lamp. These data consist of ten images taken as the stage was moved
in 2.54 mm steps in one direction across the wavefront. The analysis
consists of finding the centroid of the focal spot and its displacement
from the reference spot that would be created by a plane wave. Subsequent
experiments could include: making 2-dimensional scans; varying the radii of
curvature of the spherical wavefronts; and simulating turbulence by placing
a distorting optical element in the path of the light.